1. Field of the Invention
The present invention concerns a charging member for charging a material to be charged such as a light sensitive material for use in electrophotography and electrostatic recording processes in image forming apparatus such as electrophotographic copying machines and printers. More specifically, it relates to a charging member to be pressed on the surface of a material to be charged such as a light sensitive material and a dielectric material for uniformly charging the surface of the material to be charged, and a charging device.
2. Description of the Related Art
In electrophotographic apparatus, uniform charging is conducted to the surface of a material to be charged such as a light sensitive material and a dielectric material. A method of conducting charging by corona generated by applying a high voltage to metal wires has been adopted as a charging treatment means. However, the corona charging method involves a drawback that corona-induced products such as ozone and nitrogen oxide (NOx) formed upon charging denature the surface of the light sensitive material to deteriorate the light sensitive material or cause image blurring, and that the contamination of the wires gives an undesired effect on the image quality, leading to whitening and streaking.
In addition to the non-contact method described above, there is a contact method of conducting charging by bringing a charging member into contact with a material to be charged. The contact type charging method has an advantage that a voltage to be applied to the charging member is generally low and the amount of ozone formed is extremely small. However, it has a drawback that a toner or an external toner additive passing through a cleaning blade deposits or fuses on the charging member to cause image defects due to failed charging caused by contamination.
As a technique for improving such drawbacks, the technique disclosed in Japanese Patent Unexamined Publication 7-134467 has been known for instance.
The publication discloses a charging member in which a lubricant is contained in a surface layer formed on the surface of an elastic layer. The technique has a contamination-preventing effect due to the reduction of dynamic friction coefficient in an initial stage.
However, the technique has a problem that a hard material such as the external toner additive thrusts into the surface of the charging member during continuous use, which triggers adhesion of the toner. The surface layer of the charging member is made of a less wearing material including the technique and is formed to a thickness of about 10 .mu.m in the related art.
Therefore, the surface layer of the charging member is also contaminated during long time use, resulting in charging failure.